1. Field of the Invention
The present application relates to an internal-combustion engine.
2. Discussion of the Background
An in-cylinder direct-injection internal-combustion engine as an exemplary related art is disclosed by Japanese Patent No. 4054223. This internal-combustion engine includes a spark plug that faces the inside of a cylinder from above, a piston that reciprocates in the cylinder, a fuel injection valve that injects fuel into the cylinder from obliquely above, and so forth.
The piston has a cavity in the top surface thereof. The cavity extends from a position near the center of the piston to a position near an injection port of the fuel injection valve. The cavity has an elliptical plan-view shape whose longitudinal direction corresponds to a direction orthogonal to the axial direction of the fuel injection valve. The cavity has a flat elliptical bottom surface and guiding walls surrounding the bottom surface. The guiding walls extend linearly and obliquely upward from the bottom surface toward the outer side.
The fuel injection valve is configured to inject fuel mists from the injection port thereof. The fuel mists include a main fuel mist and a sub-fuel mist that are injected at respectively different predetermined angles. The fuel mists each have a flat shape in side view and a fan shape in plan view. The main fuel mist is injected at a nearly vertical angle and toward one of the guiding walls that is nearer to the fuel injection valve. The sub-fuel mist is injected at a nearly horizontal angle and toward the other guiding wall that is farther from the fuel injection valve.
When, for example, the internal-combustion engine is under a low load and at a low rotational speed, a stratified combustion mode is performed. In the stratified combustion mode, the fuel is injected from the fuel injection valve in the latter stage of the compression stroke. As illustrated in FIG. 6 of Japanese Patent No. 4054223, most of the main fuel mist injected as described above collides with the bottom surface of the cavity and is guided upward along the guiding wall that is nearer to the fuel injection valve, whereas most of the sub-fuel mist collides with the guiding wall that is farther from the fuel injection valve and is guided upward along the same guiding wall. Thus, a high-concentration air-fuel mixture gathers around the spark plug in a combustion chamber of the engine while a low-concentration air-fuel mixture surrounds the high-concentration air-fuel mixture, producing layers of air-fuel mixture, i.e., a stratified air-fuel mixture.